1. Field of the Invention
The invention relates to an expansion valve and a method for its control, in particular in the form of vehicle air-conditioning systems operated with CO2 as the cooling medium which have a valve housing with an inlet opening and an outlet opening and with a valve element which is displaceable out of a valve seat of a flowthrough opening, which is arranged between the inlet opening and the outlet opening, for the flowthrough of the cooling medium.
2. Related Art
For cooling medium circuits of air-conditioning systems of future motor vehicles, carbon dioxide (CO2) is preferred as the cooling medium, as this substance ensures a high degree of accident safety owing to its incombustibility and moreover is not regarded as a harmful substance for the environment. In contrast to the R134a cooling circuit, operation for CO2 cooling circuits takes place in the supercritical range as well.
An expansion valve which is used in cooling medium circuits of air-conditioning systems with CO2 is known from DE 100 12 714 A1. This expansion valve has a throttling opening with a fixed cross section in order to transfer the cooling medium from the high-pressure side to the low-pressure side for pressure relief. This cross section is always open for flowing through. If overpressure arises on the high-pressure side in the cooling medium circuit, a bypass valve connected parallel to the throttling opening is opened, so that the overpressure in excess of the optimum high pressure is reduced. The bypass valve opens only when a predetermined threshold value is exceeded on the high-pressure side.
This arrangement constitutes a functionally reliable design of an expansion valve, but it is necessary for the setting of both the threshold value and the orifice diameter to be adapted to the air-conditioning system concerned in order to achieve a maximum performance coefficient over the entire range of application of the air-conditioning system.
An expansion valve with an electronic control which has an electrically operable device for displacing a valve element is known from DE 102 19 667 A1, a further throttling location, assigned to this first throttling location in series, being provided, the passage cross section of which can be adjusted in combination with the passage cross section of the first throttling location. By virtue of this series connection of at least two throttling locations, at least one being activatable by an electric solenoid valve, the pressure difference at each individual throttling location becomes smaller than in the case of only one throttling location. This increases the control accuracy. In particular, the variations arising in the pressure difference between summer and winter can be offset.
However, this solution has the disadvantage that a costly construction is necessary. The activation of the solenoid valve requires the use of a pressure and temperature sensor or a control box with software in the control circuit, as a result of which this expansion valve is costly to manufacture and to assemble.